In association with concerns about supply of energy, shale gas has attracted attention as new energy (for example, see NPTL 1). The shale gas is natural gas contained in a shale stratum. However, the shale gas is so-called unconventional natural gas, which makes it difficult to collect the shale gas from the earth.
Accordingly, as a method of collecting the shale gas from the earth, a hydrofracturing technique has drawn attention (for example, see NPTL 2). The hydrofracturing technique is a method of artificially fracturing a reservoir rock in the vicinity of a well by applying pressure to a fracturing fluid with which the inside of the well is filled. At the time of fracturing of the reservoir rock, cracks (fractures) occur, which allows the shale gas to be collected through the cracks.
The fracturing fluid contains a plurality of particulate substances (proppants) to prevent the cracks from getting blocked after fracturing of the reservoir rock. The plurality of particulate substances are particles of sand, etc.
In the event of occurrence of the cracks, the fracturing fluid applied with pressure comes into the cracks, and accordingly the plurality of particulate substances contained in the fracturing fluid also come into the cracks. As a result, the cracks are retained as they are even if the application of pressure to the fracturing fluid is stopped.
Further, the fracturing fluid contains a viscosity-reducing agent to collect the fracturing fluid after fracturing of the reservoir rock.
To ensure that the plurality of particulate substances easily come into the cracks, the viscosity of the fracturing fluid is desirably high prior to fracturing of the reservoir rock. Meanwhile, after the plurality of particulate substances come into the cracks, to facilitate collection of the fracturing fluid with which the inside of the well is filled, the viscosity of the fracturing fluid is desirably low after the fracturing of the reservoir rock. Therefore, the viscosity-reducing agent (a breaker) having a function of reducing the viscosity of the fracturing fluid (a viscosity-reducing function) is in use.
Concerning a configuration of the viscosity-reducing agent, specific proposals have been already made. For example, to exercise the viscosity-reducing function in the middle of the use of the fracturing fluid, a viscosity-reducing agent (an encapsulated agent) having a capsule structure is in use (for example, see PTL 1). In such an encapsulated agent, a material having the viscosity-reducing function is covered with a coating film that is decomposed utilizing a hydrolysis reaction. The coating film includes poly (2-alkyl cyanoacrylate), etc. as a material to be decomposed utilizing the hydrolysis reaction.